There are various points throughout Spy Fiction where you have the chance to kill characters who still play roles in the future plot.

For example, in Case 04, A Little Bone, you have a chance to kill Nicklaus Nightwood, but this only occurs if you shoot him directly in the head. Another instance of this is in Case 06, Kill Cock Robin?, where a well placed bullet can kill Douglas Lysander as he is pacing. Killing such characters will make them (or others) spout unique quotes that you would otherwise never hear.

There are plenty more chances to kill plot important characters in the game, so, when you see them, get your firearm ready and aim for the head. Just remember, doing this will make you automatically fail your mission (rationale: you eliminated mission critical personnel/targets).

Beat the game to unlock Scarface Mode and the special option. Beat the game twice to unlock a second ending. Beat the game with Billy to unlock the 3rd costume. Beat the game with Sheila to unlock the 4th costume.